Process of coating an alloy substrate with an alloy

ABSTRACT

A process for coating an alloy to render it resistant to oxidation, sulfidation and thermal shock by forming a dispersion of an aluminum alloy and a chromium-aluminum alloy and then bonding the dispersion to the substrate.

United States Patent 1m 1111 3,720,537 Rigney lMarch 13, 1973 1 1 PROCESS OF COATING AN ALLOY [56] References Cited TR TE WITH AN ALLOY- ,SUBS A UNITED STATES PATENTS [75] Inventor: David V. Rigney, Portland, Conn.

2,621,122 12/1952 Gresham etal .l17/l3OR 3,102,044 8/1963 Joseph ..117 22 1 Asslgneel Umted Cmlmramm East 2,971,865 2/1961 Metcalfe et a1. ..117 130 R Hartford, Conn.

[22] Filed: Nov. 25, 1970 Primary Examiner-Wi1liam D. Martin Assistant Examiner-Theodore G. Davis [211 App]' 92932 Atl0rneyOwen J. Meegan [57] ABSTRACT [52] U.S. C1. ..117/50, 117/46-CA, 117/131, A process for coating an alloy to render it resistant to 117/160 R oxidation, sulfidation and thermal shock by forming a [51] Int. Cl. ..C23C 11/00, C23C 17/00 dispersion of an aluminum alloy and a chromium-alu- 1 Field 01' Search 17/130 R, 160 1 FA, 50, minum alloy and then bonding the dispersion to the 1 17/31 substrate.

6 Claims, No Drawings PROCESS OF COATING AN ALLOY SUBSTRATE WITH AN ALLOY BACKGROUND OFTHE INVENTION This invention relates to the treatment of metals and alloys to render them resistant to oxidation, sulfidation, and thermal shock. More specifically, the present invention concerns a process of forming a coating on metals and alloys and articles made therefrom. According to the present invention, a hard, high wear and oxidation resistant layer is formed on metals and alloys by applying a dispersion of powdered metals and metal alloys on the surface thereof. The coated surface is then heated to volatilize the suspending media, form a new alloy, and firmly dispose this alloy upon the article being coated.

Alloys have been developed which have special characteristics and are capable of retaining their mechanical strength at high operating temperatures. These alloys are useful in making structural parts for jet engines and gas turbines which have exacting demands made upon their strength because of elevated operating temperatures. Typical of the alloys which may be coated according to the present invention are the so called nickel-base and cobalt-base superalloys, viz. those which generally contain -25 w/o Cr, 5-l5 w/o Mo, Ta or W and 2-8 We Al and Ti. Also useful, but not possessing optimum characteristics as substrates are chromium-base and columbium-base alloys together with the iron base alloys such as tool steel.

SUMMARY OF THE INVENTION The present invention involves a slurry technique in which a coating suspension is formed of a blend of metals and alloys. The suspension is coated upon an alloy substrate, then heated to volatilize the suspending media and form an alloy thereupon. The coated article is then cooled. The product of the present process is an alloy which has a melting point far in excess of the article which serves as the substrate. When the ingredients which form the powder blend are all heated, they diffuse and react with each other and interact minimally with the substrate to form the desired coating. In the preferred embodiment, finely divided alloys of cobaltaluminum, nickel-aluminum and/or iron-aluminum are mixed with chromium-aluminum alloy. Yttrium may be added as an alloy of any of the four, but preferably as an alloy of the chromium aluminum. Because of the strong tendency of yttrium metal to oxidize, it is preferred to alloy it with other metals and alloys of the coating blend and then mix this alloy with the balance of the composition.

Also included in the blend can be copper and/or manganese, each of which helps to prevent sulfidation. The final blend can contain 20 to 45 weight percent iron, nickel and/or cobalt together with 5 to 40 weight percent chromium and 20 to 50 weight percent aluminum. Yttrium, when present, should be used in quantities between about 0.02 and 2.5 weight percent. Silicon can be introduced, if desired, in quantities less than 5 w/o to improve the coating chemistries. Copper and manganese are generally added in quantities less than about percent and preferably between about 5 and 8 percent to inhibit sulfidation as mentioned above. Generally, these metals are introduced into the blend in their elemental form, however, they may be prealloyed with the other constituents, if desired.

LII

The metallic powders, whether as alloys or elements, usually have a size of the less than 325 mesh (43 microns) although coarser particles ranging in size from about mesh (147 microns) to 325 mesh may also be used. Especially good results are achieved when the size range of the metallic particles is less than 400 mesh (38 microns) or between about 0 and 38 microns. In other words, the finer the particles the better the coatings which are produced.

In the past, the method of applying coatings to articles which utilize metal powders or mixtures of metal powders result in coatings deriving their utility solely from interdiffusion of the major components of the substrate and the applied powder. For example, Joseph (U.S. Pat. No. 3,102,044) depends on interdiffusion of aluminum and silicon with the substrate elements for primary protection characteristics. Similarly, pack cementation processes involve vapor transport of elements in the form of halides from the pack matrix to the surface of the article to be coated. Reaction of the vapors with the article produce a surface coating utilizing only the substrate components to produce the protective layer.

An additional method of applying alloyed coatings of cobalt-nickel-aluminum or iron-chromium-aluminum or nickel-chromium-aluminum is to physically deposit the alloy using vapor deposition techniques. Vapor deposition of a coating is accomplished by forming a molten pool of the coating alloy in a vacuum chamber. The parts to be coated are pre-heated to a fairly high temperature and the deposition is then accomplished by volatilizing the molten alloy onto the part. As can be readily appreciated, vacuum deposition is a technique which does not readily lend itself to high speed production. Each time a part is to be coated, it must be placed in a bell jar or chamber and a vacuum must then be drawn. Only after the vacuum is formed can the molten metal be volatilized onto the part to be coated. While such techniques do produce good coatings, they are ex pensive, time consuming to apply and they perform well only where there is no obstruction between the molten pool and all portions of the part to be coated.

A distinguishing characteristic of vapor deposited coatings is that the finished coating composition is transferred onto the substrate with very little substrate interaction. Therefore, to characterize the three methods: normal coatings produced by slurry and pack cementation methods produce coatings with major interactions with the substrate, the coatings produced by this invention involve minimal interaction with the substrate, and the vacuum deposited coatings involve only minor substrate reaction. The last two methods are particularly useful when coating articles of thin cross-section, in that the load-bearing capacity of the coated article is only slightly changed as a result of the coating process.

In the present diffusional slurry technique, a suspension of the blend is made in a lacquer-like dispersant and this suspension is placed upon a cleaned substrate by spraying, brushing, dip-coating or electrophoretic deposition. Techniques are already known for placing the metals and/or alloys in suspension and do not form a part of the present invention. To form suspensions of powders of the desired thickness and coating characteristics, certain ratios of metallic powder to liquid dispersant are used. In general, a satisfactory ratio is about 25 to 70 percent by weight (w/o) metallic powder dispersant. Many dispersants may be used in the present invention. They should, however, be readily vaporizable and may include alcohols such as methyl, ethyl, propyl and butyl alcohol, esters such as methyl, ethyl, propyl, butyl and amyl acetate, and ketones, such as acetone. Also useful are solvents such as ethylene glycol and monoethyl ether. These organic solvents are merely illustrative of those that can be used and it is to be understood that many volatile liquids will act as suitable dispersants for metallic powders. Primarily, the volatile liquid should be safe to use at room temperature, inexpensive and fluid enough to allow for spray or dip-coating on a substrate.

Frequently a binder may be added to the liquid dispersant to hold the blend of metal powders upon the substrate. When a binder is used, the powder adheres to the substrate for prolonged periods of time and thereby eliminates the necessity of baking immediately after the coating step. The binder should be substantially decomposable during sintering. Suitable binders include nitrocellulose, naphtalene, hydroxy propyl cellulose and certain stearates. Many other binders can also be used and the selection of one does not constitute a part of the present invention.

In accordance with the technique of the present invention, a dispersion of the metallic powders (having particle sizes as described previously) is mixed in the lacquer-like dispersant. Then the dispersion or slurry is then disposed upon the surface of a conventionally cleaned article to be coated by spraying, brushing, dipcoating or electrophoretic deposition. After applying the dispersion, the solvent is evaporated, thereby leaving a layer of powder on the substrate. If a binder is added to the dispersant, upon evaporation of the solvent, the binder will remain dispersed throughout the powder and will hold it to the substrate. Evaporation of the volatile solvent may be conveniently done by storing the coated substrate in the atmosphere at room temperature. When the solvent evaporates a fine layer of metallic powder is left on the surface and in any interstices, slots, holes, or other configurations of the substrate.

In general, the thickness of the coating may vary somewhat from article to article. Usually a wet thickness (before firing) of 0.003 to 0.015 inch in thickness is contemplated. Such coatings lead to a thickness, after firing, of between about 0.001 and 0.005 inch. Preferably the thickness of the coating after firing is between about 0.001 and 0.006 inch. Frequently, two and even more coats are advantageous and the invention contemplates such practices. The first coating reacts somewhat with the skin of the substrate to form a diffusion barrier layer and the second forms the oxidation resistant coating. In this manner, very oxidation resistant coatings which ruggedly adhere to the substrate are formed. When the solvent has been evaporated, the coated substrates are heated in a furnace to permanently fix the powder thereto. The furnace is maintained at the sintering temperature of the powder, that is the temperature which will permanently bond the powder to the substrate by fusion. The sintering temperature is just above the melting point of the powder and is, of course, less than the melting point of the substrate. The sintering temperature, of course, de-

pends upon the substrate and the nature of the coating metals. For chromium and nickel-base alloys, sintering temperatures between about 1,800 and 2,300F. are specially suitable. The sintering period can vary from about one-fourth to 10 hours and especially good results are achieved when the sintering is carried out for about one-fourth to 2 hours. Especially good results are obtained with a second coating upon the coated substrate and reheating this combination a second time under a firing schedule which is similar to the first. An inert or reducing atmosphere is generally preferred to sinter the coatings, however on some occasions vacuum sintering may be performed with equal suc cess.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, the following particulate alloys are mixed:

30-50 w/o Co Al (atomic basis) (46.6 w/o Co 53.4

w/o Al) 30-50 w/o Cr Al -x Y (where x 0.02 to 2.5 w/o and the quantities of Cr and A1 are in weight percent basis) with 0 to 10 weight percent each of manganese and copper and sufficient aluminum, chromium and cobalt to form a blend having the following formulation based upon weight:

20 to 45 w/o Fe, Ni, or C0 5 to 40 w/o Cr 20 to 50 w/o Al 0.02 to 2.5 w/o Y 0 to 10 w/o Cu 0 to 10 w/o Mn Nickel and iron may be substituted in whole or in part for the cobalt in the Co Al alloy so as to form an iron-aluminum or nickel-aluminum alloy or mixtures thereof. The blend is admixed with such dispersants, binders and wetting agents as are necessary to form a slurry having about 25 to w/o metallic powder, as is necessary for the particular coating operation which is utilized. When the dispersion is preformed, it is disposed on a part or substrate by the selected coating operation. The solvents are volatilized and the coating is heated, at a temperature sufficient to sinter the coating metals but insufficient to the substrate, preferably between about 1,600" and 2,300F.

While these coating compositions are particularly useful with the nickel, cobalt, iron, columbium and chromium-based alloys generally, they have been formulated to the composition having particularly good results with the nickel-base and cobalt base superalloys. The superalloys will be understood to be those strong, high-temperature materials which find particular utility in the very demanding environments such as gas turbine engines. Representative of these superalloys are those identified in the industry in the following Table 1.

TABLE 1 Alloy Composition by weight) [N 10. Cr, 15.C0, 4.5 Ti,5.5 A1.3.Mo,0.17 C,

0.75 V, 0.075 Zr, 0.015 B, balance Ni MAR-M200 9. Cr, 10. Co, 2. Ti, 5. A1, 12.5 W, 0.15 C, 1.

Nb, 0.05 Zr, 0.015 B, balance Ni W152 21. Cr, 1.75 Fe, ll. W, 2(Nb +Ta), 0.45 C,

balance Co The characteristic of the typical superalloy is its basis as a nickel-chromium or cobalt-chromium solid solution with the additions usually aluminum, titanium and/or of refractory metals for solution strengthening, and carbon, boron and zirconium to promote creeprupture ductility. Taken as a class, the super-alloys exhibit relatively good oxidation resistance at the temperatures associated with the hot section of a jet engine. However, since a compromise has normally been made in the alloy composition to achieve the best balance between strength and oxidation resistance as well as other factors, it is the usual practice to coatcertain of the components formed from these superalloys to improve their oxidation, sulfidation, erosion and thermal shock resistance, and thus to extend their operating lives.

According to the present invention, the coated article is heated to, 2',140F. for one-fourth to one-half hour, during which time sintering and diffusion takes place. Excess coating material is cleaned off the substrate and the article is recoated as described previously and reheated. A ductilization heat treatment can be made whereby the article is heated at 1,975F. for 4 to 8 hours. This treatment further diffuses the coating into the substrate and provides for homogenization of all materials in the coating. A significant improvement in thermal shock characteristics also results.

The invention will be explained further by the following examples, which though illustrative, are not intended to be limitative upon the claims.

EXAMPLE I A coating powder mixture was made containing: v pp zA s 4e.a sa.4

25ppw Cr Al Y,

2ppw Al 7 2ppw LiF (Volatilizingflux) The metals and alloys had an average particle size less than 38 a and were dispersed in ethylene glycol monoethyl ether with hydroxy propyl cellulose as a binder with a concentration of 1,220 'g/liter of metal powder. The dispersion was sprayed upon a prepared surface of nickel alloy, Bl900.'The solvent was allowed to evaporate at room temperature.

The specimen was placed within a sealed chamber. The chamber was then purged of air by following purified argon through it until the effluent showed less than 10ppm H and Sppm O prescnt. The specimen was then heated to about 2,-l40F. for-onc-fourth hour, and allowed to cool. Excess non-reacted powder was removed by liquid honing and then a second coating of the same composition was sprayed on the specimen surface. The same thermal cycle was repeated. The total coating thickness was 0.008 inch. A control specimen of the same alloy coated with a conventional aluminide (0.0025 inch thick) according to established practice was also prepared.

The two specimens were exposed to an isothermal oxidation-erosion environment at 2,100F. Each specimen was examined at intervals for coating failure. The prior art coating failed after 65 hours in test. The sample coating of this invention failed after 97 hours in test. The specific life, or life/unit thickness, for the prior art coating is 65 hours/2.5 mils or 26 hours/mil; the sample coating of this invention showed a 53.9 hours/mil (97/1 .8) specific life.

EXAMPLE ll A blend of the following was prepared and coated with the two cycle procedure described above onto B-1900 alloy except that the sinter cycles were run for one-half hour at 2,140F., and both were followed by an 8 hour heat treatment at 1,975F:

pp z s 4e.a sa.4

4ppw Co 2ppw Al I Control samples similar to those mentioned above were prepared for comparison. The specimens were 'run in a cyclic erosion sulfidation test, in which a corrosive salt (35ppw) was introduced to the burner stream. The temperature of the test cycled between 2,050E. (2 min.) 1,750F. (3 min.) and cooling (2 min). The prior art coatings, each 0.0035 inch thick, failed after 61.75 hours and 66.9 hours for specific lives of 17.6 and 19.1 hours/mil respectively; the specimens prepared according to Example [I were 0.0037 and 0.0041 inch thick, and exhibited a total life of 92.2 hours each, for specific lives of 24.6 and 22.5 hours/mil respectively.

B1900 alloy airfoils were also coated in the same manner as described above and subjected to comparative testing in thermal shock against the prior art coating. Specimens'with trailing edge dimensions of 0.040 inch showed failure of the prior art coated specimen at about 3,400 cycles; with the coating of this example, failure occurred at about 2,700 cycles. Specimens of the prior art coating with trailing edge dimensions of 0.046 inch show failures near 1,300 cycles; those specimens having the same dimensions coated according to the example were discontinued after 5,000 cycles.

EXAMPLE III when the standard prior art coating showed a normalized specific life of 20 hours/mil.

EXAMPLE IV A blend of the following was prepared and coated as described in Example I:

The samples were heated for two hours at 2,140F.

sistant to oxidation, sulfidation, and thermal shock, the coating having a melting point higher than that of the substrate, which comprises:

forming a slurry consisting essentially of about, by

and the specific life (std 20 hours/mil) was 45.8 5 weight, 30-50 percent C Al 30-50 percent hours/mil for one sample and 37.1 hours/mil for C AI Y where x ()2 2 5 d h mp another Sample in a Vane cyclic Sulfidation testties of Cr and Al are expressed on a weight basis, in

a volatile dispersant; EXAMPLE v applying the slurry to the substrate;

A blend f the f ll i was prepared and coated as volatilizing the dispersant and heating the substrate described in Example 1; to a temperature, below the melting temperature 50ppw Co Al (+1 w/o Y) of the substrate, to form the protective alloy coat- 50 CtmAiGo ing and bond it to the substrate; and

g Cu cooling the coated substrate. I

h Sample was h d at 2 140? f 40 minutes 2. The process according to claim 1 wherein the and tested in vane cyclic sulfidation test for specific life dispersion contains, by weight, less than 10 percent against a standard having a specific life of hours/mil. Copper, less than l0 percent manganese and less than 5 The sample had a specific life of 39.1 hours/mil. percent silicon.

Additional Examples of the present invention are 3. The process according to claim I wherein the slurgiven in the following Table. ry as applied, is between about 0.003 and 0.015 inch TABLE II [Isothermal Oxidation (2,100 F.)]

'lotallifn Specific life Sample Substrate Coating, partshywr-iglit Total livatti'i'atinu (hours) (hours/mil) l... 13-15100 llit)l1trl. 4lii's.atl,!l75 F... ill 2% 2 n-iiinu mom/iii, 4m), SUUI'mAl Yi, so, an", M1, Mr t I04 40 1 lWA-47. 2 Total of two (-natiiig (.ytll'S.

TABLE III thick.

4. The process according to claim 1 wherein a Vane Cyclic Sulfidation second coating of the same composition as the first Tom] coating is applied to the coated and heated article and Sample Alloy Coating Tomi Hem Lif the second coating is heated to sinter the alloy whereby I B4900 g i gg ih 2 the first coating forms a diffusion barrier and the 8 2 A] 2,170? second coating forms an oxidation resistant coating.

2 34900 50 CPAIY, 5A1, min at 86 5. The process according to claim 1 wherein the 5 5 heating is at a temperature between about 1 800 and 50 CrAl, l0 Cr, 40

3 Wl-52 10 Cu, 40 COM, 20 min at 132 2,300F.

i gbg t 6. A process for forming a high melting point protec- 4 wl-52 50 CrAlY,5Co, 30 min at 232 tive alloy coating consisting essentially of chromium, l8 33 A] aluminum and yttrium in base metal selected from the 5 MAR-M302 IOCo, 10 Cr, lhour at 232 group consisting cobalt, iron and nickel, on an alloy 18%;? 2 Al substrate to render it resistant to oxidation, sulfidation 6 MAR-M301): l0Co,l0Cr, lhour at 175 and thermal shock, the alloy coating comprising an 70 2 M 3 29 Se uemi overlay on the substrate and having a melting point in Standard stan ard at 37,6 5 5 6 excess of that of the substrate, the coating being 7 [H900 g ggk 133 I08 formed in situ on the substrate surface and being sub- 4 SOCYMY' 2,170? stantially independent of the substrate constituents for 8 Co, 2 Mn its protective function, which comprises the steps of:

8 B4900 g fgk 30 min a 108 forming a selectively prealloyed dispersion consisting 2A1, 50 CrAlY, 2,l25F. essentially of a first alloy of aluminum with the base metal of the coating and a second alloy of Two Coats chromium and aluminum, the dispersion also including yttrium in alloyed form with at least one of the coating constituents, in a volatile suspending media, the mixture of the alloys in the dispersion having a melting point as applied less than that of the substrate;

depositing the dispersion upon the substrate;

volatilizing the suspending media and heating the coated substrate to a temperature between the melting points of the substrate and that of the alloys in the dispersion to form in situ the protective alloy coating and bonding it to the substrate; and cooling the coated substrate. 

1. A process for depositing a protective cobalt-chromium-aluminum-yttrium alloy coating on an alloy substrate selected from the group consisting of the nickel-base and cobalt-base superalloys to render it resistant to oxidation, sulfidation, and thermal shock, the coating having a melting point higher than that of the substrate, which comprises: forming a slurry consisting essentially of about, by weight, 30-50 percent Co2Al5, 30-50 percent Cr40Al60-x Yx, where x 0.02-2.5 and the quantities of Cr and Al are expressed on a weight basis, in a volatile dispersant; applying the slurry to the substrate; volatilizing the dispersant and heating the substrate to a temperature, below the melting temperature of the substrate, to form the protective alloy coating and bond it to the substrate; and cooling the coated substrate.
 2. The process according to claim 1 wherein the dispersion contains, by weight, less than 10 percent copper, less than 10 percent manganese and less than 5 percent silicon.
 3. The process according to claim 1 wherein the slurry as applied, is between about 0.003 and 0.015 inch thick.
 4. The process according to claim 1 wherein a second coating of the same composition as the first coating is applied to the coated and heated article and the second coating is heated to sinter the alloy whereby the first coating forms a diffusion barrier and the second coating forms an oxidation resistant coating.
 5. The process according to claim 1 wherein the heating is at a temperature between about 1,800* and 2,300*F. 